This invention relates to an electric arrangement for igniting and supplying a gas discharge lamp, which arrangement is intended to be connected to an alternating voltage source and comprises a rectifier bridge connected to a DC/DC converter provided with a rectifier element, a coil and a high-frequency switched semiconductor switching element coupled to a drive circuit, said DC/DC converter being connected to the input terminals of a high-frequency DC/AC converter incorporating the lamp and provided with semiconductor switching elements. A capacitor is arranged between said input terminals, and a sensor for measuring the current taken off by the converter is connected between one of the input terminals and a semiconductor switching element of the DC/AC converter.
An arrangement of this type is described in British Patent Application 2,016,222 A laid open to public inspection.
This Patent Application describes a power supply circuit including a DC/DC converter, e.g. a forward converter, which is coupled to a high-frequency DC/AC converter. The DC/DC converter operates as a current source for the high-frequency switching DC/AC converter coupled thereto. A square-wave current is applied to the lamp by means of the latter converter. The circuit also includes a sensor with which the current intensity of the lamp is measured and compared with a fixed reference current by means of a control circuit coupled to the sensor. The control circuit ensures, in conjunction with a drive circuit which is coupled thereto, and which serves to control the semiconductor switching element in the forward converter, that the said switching element is rendered conducting and non-conducting in such a way that the current intensity supplied to the lamp is set to a predetermined value.
However, a drawback of the known circuit is that the power consumption of the lamp, and hence the light output, decrease when operating the lamp at a relatively low lamp voltage (for example, due to ageing or, in the case of a low-pressure mercury vapour discharge lamp, operation in a relatively hot location). Even when placing in the circuit a low-pressure mercury vapour discharge lamp with a rare gas mixture present in the lamp vessel in a composition which deviates from the conventional composition and results in a like deviation of the operating voltage, it has been found that the light output of such a lamp decreases to an unacceptably low level.